Low loss optical fiber and method for making same

Abstract

A method of forming an alkali metal oxide-doped optical fiber by diffusing an alkali metal into a surface of a glass article is disclosed. The silica glass article may be in the form of a tube or a rod, or a collection of tubes or rods. The silica glass article containing the alkali metal, and impurities that may have been unintentionally diffused into the glass article, is etched to a depth sufficient to remove the impurities. The silica glass article may be further processed to form a complete optical fiber preform. The preform, when drawn into an optical fiber, exhibits a low attenuation.

We claim:  
     
         1 . A method of forming an optical fiber precursor comprising: 
 diffusing an alkali metal through a surface of a glass article, said glass article being comprised of at least 80 mole percent SiO2; and    etching the diffusion surface of said glass article, to a depth sufficient to remove diffused impurities, to form an optical fiber precursor.    
     
     
         2 . The method of  claim 1  wherein said glass article in said diffusing step comprises less than about 100 ppm chlorine.  
     
     
         3 . The method of  claim 1  wherein said glass article in said diffusing step comprises less than about 50 ppm chlorine.  
     
     
         4 . The method of  claim 1  wherein said glass article in said diffusing step comprises less than about 100 ppb  − OH.  
     
     
         5 . The method of  claim 1  wherein said glass article in said diffusing step comprises less than about 20 ppb  − OH.  
     
     
         6 . The method of  claim 1  wherein said alkali metal in said diffusing step is K, Na, Li, Cs or Rb.  
     
     
         7 . The method of  claim 6  wherein prior to said diffusing step said alkali metal is derived by heating a bromide of K, Na, Li, Cs or Rb.  
     
     
         8 . The method of  claim 6  wherein prior to said diffusing step said alkali metal is derived by heating an iodide of K, Na, Li, Cs or Rb  
     
     
         9 . The method of  claim 6  wherein prior to said diffusing step said alkali metal is derived by heating a fluoride of K, Na, Li, Cs or Rb  
     
     
         10 . The method of  claim 1  wherein said glass article in said diffusing step is a glass tube, and wherein said alkali metal is diffused through the inside surface of said glass tube.  
     
     
         11 . The method of  claim 10  wherein said diffusing step comprises diffusing said alkali metal to a depth of at least about 100 microns from the inside surface of said glass tube.  
     
     
         12 . The method of  claim 10  wherein said diffusing step comprises diffusing said alkali metal to a depth of at least about 300 microns from the inside surface of said glass tube.  
     
     
         13 . The method of  claim 10  wherein said diffusing step comprises diffusing said alkali metal to a depth of at least about 500 microns from the inside surface of said glass tube  
     
     
         14 . The method of  claim 1  wherein said glass article in said diffusing step is a glass rod, and wherein said alkali metal is diffused through the outside surface of said glass rod.  
     
     
         15 . The method of  claim 14  wherein said diffusing step comprises diffusing said alkali metal to a depth of at least about 100 microns from the outside surface of said glass rod.  
     
     
         16 . The method of  claim 14  wherein said diffusing step comprises diffusing said alkali metal to a depth of at least about 300 microns from the outside surface of said glass rod  
     
     
         17 . The method of  claim 14  wherein said diffusing step comprises diffusing said alkali metal to a depth of at least about 600 microns from the outside surface of said glass rod  
     
     
         18 . The method of  claim 10  further comprising 
 collapsing said glass tube to form an intermediate glass article;  
 adding additional glass to said intermediate glass article to form a complete optical fiber preform; and  
 drawing said optical fiber preform into an optical fiber.  
 
     
     
         19 . The method of  claim 14  further comprising 
 adding additional glass to said glass rod to form an optical fiber preform; and  
 drawing said optical fiber preform into an optical fiber.  
 
     
     
         20 . The method of  claim 18  further comprising inserting a glass rod into said glass tube prior to said collapsing step.  
     
     
         21 . The method of  claim 1  wherein said etching step comprises etching away glass from the diffusion surface of said glass article to a depth of at least about 5% of the diffusion depth of the alkali metal.  
     
     
         22 . An optical fiber wherein the core is comprised of SiO 2  and an alkali metal oxide of the form X 2 O where X is selected from the group consisting of K, Na, Li, Cs and Rb and 0.035≦X 2 O≦6 mole percent and wherein said core is further comprised of less than about 100 ppm chlorine.  
     
     
         23 . The optical fiber of  claim 22  wherein said core is comprised of less than about 50 ppm Cl.  
     
     
         24 . The optical fiber of  claim 22  wherein said core of said optical fiber is further comprised of less than about 100 ppb  − OH.  
     
     
         25 . The optical fiber of  claim 22  wherein said core of said optical fiber is comprised of less than about 20 ppb  − OH.